Sheet reversing apparatus and image forming apparatus

ABSTRACT

In a sheet reversing unit, an attraction belt is supported, in a 180° twisted position, between a belt driving roller at a conveying start position and a belt driving roller at a conveying end position, and a sheet is conveyed by providing the attraction belt with an electrostatic force from an AC power supply and thus by being attracted to the attraction belt, whereby the sheet is reversed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present-invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, and more particularly to an apparatus for reversing a sheet in case of forming images on both sides of a sheet.

2. Related Background Art

There is known an image forming apparatus such as a copying -apparatus, a printer or a facsimile apparatus, that is provided with a mode of forming images on both front and rear sides of a sheet (hereinafter called both-side image forming mode). In such both-side image forming mode, image formations on both sides of a sheet are achieved by reversing a sheet by a sheet reversing apparatus provided in the image forming apparatus.

FIG. 10 is a lateral cross-sectional view showing a configuration of a sheet reversing apparatus of a prior switchback type provided for the both-side image forming mode.

A sheet reversing apparatus 100 b is provided with a conveying guide 101 for receiving a sheet from an image forming apparatus 100 a, a conveying guide 103 for discharging the sheet after image-formation to the exterior of the apparatus, and conveying guides 104, 107, 110 for reversing the received sheet.

The conveying guide 101, receiving the sheet from the image forming apparatus 100 a, is branched to the conveying guide 103 or to the conveying guide 104, and a flapper 108 is provided at a branching point for controlling the advancing direction of the sheet, which is advanced by conveying rollers 102.

The conveying guide 104 is provided with a branching point to the conveying guide 107 and to the conveying guide 110, and a flapper 109 is provided at the branching point for controlling the advancing direction of the sheet, which is advanced by reversing rollers 105. The conveying guide 110 is provided with conveying rollers 106, which advances a reversed sheet toward the image forming apparatus 100 a.

In the following, there will be explained a sheet reversing process in the prior sheet reversing apparatus 100 b. A sheet, after an image formation and an image fixation on a side in the image forming apparatus 100 a, is conveyed through the conveying guide 101 to the sheet reversing apparatus 100 b. In this state, the flapper 108, provided at the branching point of the conveying rollers 102, opens the conveying guide 104 and closes the conveying guide 103. Therefore, the sheet is conveyed to the conveying guide 104 by the rotation of the conveying rollers 102.

When the sheet is conveyed to the conveying guide 104, the flapper 109 provided at the branching point of the reversing rollers 105 closes the conveying guide 110, so that the sheet passes through the conveying guide 104 and is advanced by the reversing rollers 105 to the conveying guide 107. The reversing rollers 105 are stopped when a rear end of the sheet passes through the flapper 109.

After the reversing rollers 105 are stopped, the flapper 109 closes the conveying guide 104 and opens conveying guide 110. Then the reversing rollers 105 are rotated in a reverse direction to advance the sheet to the conveying guide 110, and the sheet is fed by the conveying rollers 106 to the image forming apparatus 100 a. In case of thereafter forming an image on the rear side of the sheet in the both-side image forming mode, the above-described operations are repeated.

In such sheet reversing apparatus of switchback type, it is necessary to stop and reverse the rollers for sheet switchback, thus resulting in a waste in time.

Also a guide member for temporarily holding the sheet at the sheet switchback is occupied until the entire sheet passes through such guide, so that a longer time is required for a longer sheet and the productivity at the both-side printing process is difficult to improve.

For avoiding such drawbacks, a sheet reversing apparatus of another type is disclosed in Japanese Patent Application Laid-open No. 11-207927 or Japanese Patent Application Laid-open No. 2002-20000 (USP 2002069774A).

In an invention disclosed in Japanese Patent Application Laid-open No. 11-207927, a sheet is nipped by a sheet conveying apparatus having nipping claws, and such sheet conveying apparatus is guided along a conveying path having a shape of a Mebius' belt to reverse the sheet.

However, in such sheet reversing apparatus in which the sheet is guided along a conveying path having a shape of a Mebius' belt, since the sheet is nipped by the nipping craws, a trace of the nipping claws is formed on the sheet surface and the sheet tends to cause, in the course of conveying, a flapping in a portion other than the part nipped by the nipping claws thereby causing a damage on the sheet.

Also in an invention disclosed in Japanese Patent Application Laid-open No. 2002-20000, two belts 7, 8, each crossing by a turn by 180°, are provided between deflecting guide rollers 3, 4, 5, 6, and a sheet is nipped between the two belts 7, 8 and is conveyed under a turn by 180°.

Also in such configuration, since the sheet S is nipped and conveyed only by the two belts 7, 8, it is necessary to increase the nipping force of the belts 7, 8 in order to achieve a conveying without sheet aberration, and such increased nipping force may result in a trace of the belt on the sheet. Also the sheet tends to cause, in the course of conveying, a flapping in a portion other than the part nipped by the belt thereby causing a damage on the sheet.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a sheet reversing apparatus capable of improving a reversing process for a sheet and of attaining a stable sheet reversing without a damage to the sheet.

The aforementioned object can be attained, according to the present invention, by a sheet reversing apparatus including a conveying belt for conveying a sheet, drive means which drives the conveying belt, and attraction means which attracts the sheet to the conveying belt, wherein a sheet attracting surface of the conveying belt has a twist between a conveying start position and a conveying end position, whereby a conveyed sheet is reversed by a predetermined angle.

In the sheet reversing apparatus and the image forming apparatus of the present invention, since a surface of a conveying belt is twisted by a predetermined angle between a conveying start position and a conveying end position and a sheet is conveyed by being attracted to such conveying belt, it is possible to reduce a time required for sheet reversing, thereby improving the productivity at the both-side image forming process. Also as the sheet is not locally nipped but entirely attracted in the conveying, in comparison with a prior apparatus in which the sheet is reversed along a conveying path formed in a shape of a Mebius' belt, it is rendered possible to avoid a print quality deterioration such as a trace of nipping on the sheet. Also the stability in sheet conveying can be improved as a flapping of the sheet at the conveying can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral cross-sectional view of an image forming apparatus embodying the present invention;

FIG. 2 is a perspective view of a sheet reversing apparatus of the present invention as a reversing unit;

FIG. 3 is a plan view of a reversing unit embodying the present invention;

FIG. 4 is a lateral view of a reversing unit embodying the present invention;

FIG. 5 is a lateral view showing a conveying start position in the reversing unit embodying the present invention;

FIG. 6 is a lateral view showing a conveying end position in the reversing unit embodying the present invention;

FIG. 7 is a lateral view showing another sheet separating method from an attraction belt 33 of the embodiment;

FIG. 8 is a perspective of the reversing unit of the embodiment in the course of sheet conveying;

FIG. 9 is a perspective view showing another example of the reversing unit of the embodiment;

FIG. 10 is a lateral cross-sectional view of a sheet reversing apparatus of prior switchback type; and

FIG. 11 is a perspective view showing another prior sheet reversing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, a sheet reversing apparatus and an image forming apparatus embodying the present invention will be explained with reference to FIGS. 1 to 8.

FIG. 1 is a lateral cross-sectional view of an image forming apparatus embodying the present invention; FIG. 2 is a perspective view of a sheet reversing apparatus of the present invention as a reversing unit; FIG. 3 is a plan view of a reversing unit embodying the present invention; FIG. 4 is a lateral view of a reversing unit embodying the present invention; FIG. 5 is a lateral view showing a conveying start position in the reversing unit embodying the present invention; FIG. 6 is a lateral view showing a conveying end position in the reversing unit embodying the present invention; FIG. 7 is a lateral view showing another sheet separating method from an attraction belt 33 of the embodiment; and FIG. 8 is a perspective of the reversing unit of the embodiment in the course of sheet conveying.

The image forming apparatus 1 is constituted of an original reading portion 2 for reading an original such as a book or a document as character data or image data, an image forming portion 3 for forming an image of the image data, read by the original reading portion 3, by a transfer on a sheet (copy paper or glossy paper), a feeding portion 5 for feeding he sheet to be used in the image forming portion 3, a fixing portion 6 for fixing a toner image, transferred onto the sheet in the image forming portion 3, and a both-side portion for executing a sheet reversing in case of a both-side printing.

The original reading portion 2 is provided with an optical scanning portion 10 having a light source, which illuminates an original placed on an original plate 8. A reflected light generated by such illumination is reflected in succession by mirrors provided in the original reading portion 2, then guided through a reducing lens 11 and read by an image sensor such as CCD 12. The light read by the CCD 12 is photoelectrically converted, and image data thus obtained are subjected to an A/D conversion and an image processing, and are outputted to a memory or a hard disk (not shown).

The image forming portion 3 is provided with a laser unit 13 which emits a laser beam under the control of a laser driver, according to the image data read by the original reading portion 2. The laser beam falls on an unillustrated polygon mirror, and scans a photosensitive drum 15 along a generating line thereof, by the rotation of the polygon mirror. Such scanning exposure of the surface of the photosensitive drum 15 which is charged in advance by a charger 16 forms an electrostatic latent image.

The electrostatic latent image is developed on the photosensitive drum by a developing device 17, and a resulting toner image is transferred onto a sheet by a transfer charger 19. A residual toner, remaining on the photosensitive drum 15 after the image transfer, is removed by a cleaning device 20. The charger 16, the developing device 17, the transfer charger 19 and the cleaning device 20 are positioned close to the photosensitive drum 15 and arranged in this order along the rotating direction of the photosensitive drum 15.

In the image forming apparatus 1, mechanisms for four colors of cyan, magenta, yellow and black are arranged in succession and images formed according to color-separated respective image data are transferred in superposition to obtain a color image.

In the feeding portion 5, there is provided a sheet cassette 21 for stacking sheets, which is detachably mounted in a lower part of the image forming apparatus 1. In a position close to the sheet cassette 21, there is provided a pickup roller 22, and an unillustrated solenoid connected thereto is turned on in a stand-by state for sheet feeding, whereby the pickup roller 22 is separated from the sheet surface.

At a sheet feeding operation, the solenoid is turned off to contact the pickup roller 22 with an upper most sheet. The sheet in contact with the pickup roller 22 is fed by the driving of the pickup roller 22. In this state, the pickup roller 22 is rotted by a driving power transmitted from a conveying roller 23 through a timing belt. The sheet driven by the pickup roller 22 is nipped and conveyed by the conveying roller 23 and a retarding roller 25. The conveying roller 23 is rotated in a conveying direction of the sheet, while the retarding roller 25 is rotated, through an unillustrated torque limiter, in a direction opposite to the conveying direction of the sheet.

Thus, when only one sheet is conveyed, since only one sheet is nipped between both rollers, a frictional force between the sheet and the roller overcomes a driving force of the torque limiter for the retarding roller 25, whereby the retarding roller rotates in the conveying direction.

In case two or more sheets are advanced in superposition, upon reaching a nipping position between the conveying roller 23 and the retarding roller 25, a frictional force between the sheets becomes weaker than the power of the torque limiter, whereby the retarding roller 25 rotates in a direction opposite to the conveying direction and an uppermost sheet only among the plural sheets is separated and fed in advance.

Thus, in case the pickup roller 22 advances plural sheets at the same time, the aforementioned function allows to separate and advance the uppermost sheet only among such plural sheets, whereby the sheets stacked in the sheet cassette 21 can be conveyed one by one. Then, thus separated sheet is conveyed to a sheet registration portion 9.

The fixing portion 6 is provided with a fixing roller 26 and a pressure roller 27 for fixing a toner image onto a sheet, and, when a sheet bearing a transferred toner image is conveyed by a conveyor belt 32 from the image forming portion 3, the fixing roller 26 and the pressure roller 27 apply heat and pressure thereto thereby fixing the toner image onto the sheet.

After such fixing processing in the fixing portion 6, in case of an ordinary image forming mode (one-side image forming mode), the sheet is discharged by paired discharge rollers 4 to the exterior of the image forming apparatus 1.

On the other hand, in case of a both-side image forming mode, the sheet is conveyed, after passing the fixing roller 26 and the pressure roller 27, to the both-side portion 7.

The sheet conveyed into the both-side portion 7 passes a both-side conveying path 29 and introduced into a both-side unit (sheet reversing apparatus) 30. It is then conveyed by re-feeding rollers 31 to the sheet registration portion 9 for repeated image formation, and is thereafter discharged to the exterior of the image forming apparatus 1 in the same manner as in a one-side image forming mode.

In the following there will be explained the reversing unit 30, serving as a sheet reversing apparatus in the present embodiment.

In the reversing unit 30, an attraction belt 33, serving as a conveying belt of the invention for attracting and conveying a sheet, is supported between a belt driving roller 35 at a conveying start position and a belt driving roller 36 at a conveying end position. A sheet attracting surface of the attraction belt 33 at the position of the belt driving roller 35 is in a position twisted by 180° from a position of a sheet attracting surface of the attraction belt 33 at the position of the belt driving roller 36. More specifically, the sheet attracting surface of the attraction belt 33 at the position of the belt driving roller 35 is faced downwards, while the sheet attracting surface of the attraction belt 33 at the position of the belt driving roller 36 is faced upwards.

In the present embodiment, the sheet attracting surface is twisted by 180°, but such angle can be suitably selected according to the requirement.

Also at the conveying start position of the sheet, a pressing roller 39 is opposed to the belt driving roller 35 across the attraction belt 33. The pressing roller 39 applies a certain biasing force to the belt driving roller 35, whereby the sheet conveyed to the reversing unit 30 is attracted by the attraction belt 33. The biasing force applied to the belt driving roller 35 may be controlled in a magnitude and a method of application for example by a solenoid or a motor, depending on a type of the sheet.

Also a backup roller 37 is provided at an approximate center of the belt driving roller 35 and the belt driving roller 36 and inside the supported attraction belt 33, in order to avoid mutual friction thereof.

Though omitted in the present embodiment, the belt driving rollers 35, 36 as the drive means of the invention are driven by drive means such as a motor to rotate the attraction belt 33 at an appropriate speed. Also a conveying property of the belt may be improved for example by forming ribs on end portions of the attraction belt 33 or by providing either of the belt driving rollers 35, 36 with a steering mechanism for regulating an alignment of the attraction belt 33.

On the other hand, for attracting the sheet onto the attraction belt 33, there is employed means which charges the attraction belt electrostatically. For electrostatic charging of the attraction belt 33, a charge control portion 40, serving as control means of the invention as shown in FIG. 2, controls an AC power supply 41 to supply the belt driving rollers 35, 36 and the backup roller 37 with a current thereby providing the attraction belt 33 with an electrostatic force. It may be used in DC power supply to supply the belt driving rollers 35, 36 and the backup roller 37 with a current thereby providing the attraction belt 33 with an electrostatic force.

The current is supplied to the belt driving rollers 35, 36 and the backup roller 37 as explained above for obtaining an electrostatic force, but the current need not be supplied to all these members as long as an electrostatic force of a necessary attractive force can be obtained in the attraction belt 33.

Also the attractive force required for sheet attraction is variable depending on the type of the sheet (for example a weight per unit area, a stiffness, a thickness etc.). It is therefore possible to reduce the electric power consumption by controlling, in the charge control portion 40, the attractive force according to the type of the sheet to be used. It is also possible to reduce the electric power consumption by identifying the conveyed sheet by an unillustrated sensor or the like and controlling the attractive force for each sheet.

In the following, there will be explained functions of the reversing unit 30 of the present embodiment.

A sheet is conveyed between entrance conveying guides 42 a, 42 b, and is attracted under the attraction belt 33 by a charging function of the belt driving roller 35. The sheet attracted to the attraction belt 33 is guided to the conveying end position along the twisted form of the attraction belt 33.

At the conveying end position of the reversing unit 30, exit conveying guides 43 a, 43 b are provided in opposed positions as shown in FIG. 6. The exit conveying guide 43 a is provided with an overlapping for a length L, in order to prevent that an end portion of the sheet, when separated from the attraction belt 33, springs up to cause a defective sheet transfer.

The sheet is separated from the attraction belt 33 by the stiffness only of the sheet attracted on the attraction belt 33, utilizing a curvature of the attraction belt 33 at the conveying end position, determined by the external radius of the belt driving roller 36 and a bending resistance of the sheet (separation by curvature). More specifically, the sheet attracted and conveyed by the attraction belt 33 has a stiffness larger than the attractive force of the attraction belt, so that the sheet cannot turn at the conveying end position and is separated from the attracting surface of the attraction belt 33.

FIG. 7 shows another example of method for separating the sheet from the attraction belt 33.

A charge eliminating needle 45 executes a separation by reducing the attractive force of the attraction belt 33, and a separating claw 46 forcedly separates the sheet and the attraction belt 33 by a claw. The separating claw 46 is provided rotatably about a rotary center 46 a and is contacted with the attraction belt 33 by an unillustrated torsion spring.

Thus, such separating method with the charge eliminating needle 45 and the separating claw 46, or by separation by curvature allows to reverse the sheet without causing a change in the conveying speed of the attraction belt 33 and the sheet.

FIG. 7 shows a case where the charge eliminating needle 45 and the separating claw 46 are employed in combination, but they need not be employed in combination but either one may be employed singly. Also the charge eliminating needle 45 and the separating claw 46 may be employed as auxiliary means for the aforementioned separation by curvature.

FIG. 8 is a perspective view in a state of actual sheet conveying by the reversing unit 30 explained in FIGS. 2 to 7. As shown in FIG. 8, a sheet S is conveyed, with a top surface (surface of first image formation of the sheet) upwards, from the entrance conveying guide 42 a to the reversing unit 30.

The conveyed sheet S is conveyed by the attraction belt 33, which is driven by a rotation of the belt driving roller 35 in a direction A in FIG. 2 and by a rotation of the belt driving roller 36 in a direction B shown in FIG. 3. In this state, the backup roller 37 is driven in a direction C shown in FIG. 2, and the pressing roller 39 is rotated in a direction D shown in FIG. 2. The sheet S is attracted, at the top surface thereof, to the attraction belt 33 and is guided in this state to the conveying end position along the twisted structure of the attraction belt 33.

Then the sheet S is separated by the aforementioned separating method of the attraction belt 33 and the sheet, and is transferred from the exit conveying guides 43 a, 43 b to a next conveying path. As the attraction belt 33 constituting the reversing unit 30 is twisted in the surfaces thereof by 180° between the conveying start position and the conveying end position, the sheet is reversed by between the top surface and the rear surface thereof by being conveyed by such attraction belt 33. A twisting direction and a conveying direction of the attraction belt 33 are preferably so selected that the sheet S is brought to a front portion of the apparatus.

In the following, there will be explained another example of the reversing unit of the present embodiment. FIG. 9 is a perspective view showing another example of the reversing unit of the present embodiment, wherein compositions same as those in FIG. 1 are represented by symbols same as those in FIG. 1 and explanations in FIG. 1 are applicable.

FIGS. 1 to 8 have explained a method of reversing a sheet by a single attraction belt 33. It is however not necessary to reverse the sheet by a single attraction belt 33, but a similar effect can be obtained by a reversing divided in two portions as shown in FIG. 9.

In a reversing unit 49 shown in FIG. 9, a sheet S is conveyed from a direction E, and is subjected to a reversing by 90° by an attraction belt 49 of a first reversing unit 47 a. After passing the first reversing unit 47 a, the sheet S is conveyed through an intermediate path 52, under supporting by conveying rollers 51. Then, upon reaching a second reversing unit 47 b, it is further subjected to a reversing process by 90° by an attraction belt 50. Then, after a reversing by 180° in total, the sheet is discharged in a direction F and is conveyed again to the sheet registration portion 9.

Such division of the reversing process in two portions allows to reduce the distance required for reversing, whereby the apparatus can be made more compact. The aforementioned reversing unit 47 shows a configuration of dividing the attraction belt into two portions, but such division is not limited to two portion but may be made in three or more portions. Thus, such divided configuration of the reversing unit allows to enjoy the effect of the present invention without adopting a large-sized belt.

In the reversing unit 30 of the present embodiment, as explained in the foregoing, the attraction belt 33 for conveying the sheet is supported, in a twisted shape by 180°, between the belt driving rollers 35 and 36, and the sheet is conveyed by being attracted on such attraction belt 33, whereby the time required for sheet reversing can be reduced to improve the productivity at the both-side printing process.

Also the reversing unit 30 of the present embodiment can reduce the time required for sheet reversing by attracting the sheet to the attraction belt 33 and conveying the sheet under twisting of the attraction belt 33 by a predetermined angle between the conveying start position and the conveying end position, whereby it is unnecessary to convey the sheet with an increased speed, thereby reducing a stain on the sheet by a roller or the like and avoiding a deterioration in the quality of the sheet.

Also in the reversing unit 30 of the present embodiment, the sheet, being conveyed in a state attracted to the attraction belt 33, is not supported locally and is prevented from a damage on the sheet such a nipping trace thereon and from a deterioration in the print quality, and is further prevented from a sheet flapping during the conveying operation, thereby improving the stability of the conveying.

Also the reversing unit 30 of the present embodiment, by conveying the sheet in a state attracted to the attraction belt 33, can reduce the time required for reversing the sheet, and does not require an increased conveying speed in the both-side portion 7, whereby a motor of a smaller driving power can be employed thereby suppressing a temperature elevation in the apparatus or a noise level.

Furthermore, the reversing unit 30 of the present embodiment can reduce an electric power consumption by controlling the electrostatic force, employed for attracting the sheet to the attraction belt 33, according to the type of the sheet by the charging control portion 40.

Furthermore, the reversing unit 30 of the present embodiment, in which the sheet is conveyed by being attracted to the attraction belt 33 of which the surface is twisted by a predetermined angle between the conveying start position and the conveying end position, can save the space required for inverting the sheet in comparison for example with the sheet reversing apparatus of switchback type, thereby compactizing the sheet reversing apparatus and the image forming apparatus utilizing the same.

In the foregoing examples of reversing the sheet with a single attraction belt 33 and with two attraction belts 49 and 50, the reversing angle is assumed as 180°, but such angle is not limited to 180° and can be selected arbitrarily according to the process on the sheet after the reversing or to the shape of the conveying path. This also applied to a case where three or more attraction belts are employed.

Furthermore, in the foregoing examples, the image forming apparatus 1 having the reversing unit 30 (sheet reversing apparatus) is explained as a color image forming apparatus, but it can also be an exclusively monochromatic image forming apparatus as long as the both-side printing can be executed on the sheet.

Furthermore, in the foregoing examples, the image forming apparatus 1 having the reversing unit 30 is assumed to execute an image formation based on the image data read by the original reading portion 2, but there can also be utilized an image forming apparatus capable of network connection, such as an image forming apparatus executing the image formation based on image data transmitted from an information terminal such as another image forming apparatus or a personal computer through a network such as an internet, a LAN or a wireless LAN.

Also in the foregoing example, the backup roller 37 for avoiding mutual friction of the attraction belt 33 is employed by only one unit, but it may be used in plural units to further stabilize the shape of the attraction belt 33 at the sheet conveying and the conveying operation thereof.

As explained in the foregoing, the sheet reversing apparatus and the image forming apparatus of the present invention is effective in executing both-side printing of a sheet, and particularly in case a high speed is required in the both-side image forming process.

This application claims priority from Japanese Patent Application No. 2004-291404 filed on Oct. 4, 2004, which is hereby incorporated by reference herein. 

1. A sheet reversing apparatus for reversing a sheet by a predetermined angle, comprising: a conveying belt for conveying a sheet; drive means which drives the conveying belt; and attraction means which causes the conveying belt to attract the sheet; wherein a sheet attractive surface of the conveying belt has a twisting between a conveying start position and a conveying end position.
 2. A sheet reversing apparatus according to claim 1, wherein the conveying belt reverses a top side and a rear side of the sheet between the conveying start position and the conveying end position.
 3. A sheet reversing apparatus according to claim 1, wherein the conveying belt includes at least two conveying belts for conveying the sheet along a conveying direction thereof; and a sum of twisting angles of the plural conveying belts is equal to an angle required for reversing a top side and a rear side of the sheet at the conveying start position.
 4. A sheet reversing apparatus according to claim 1, wherein the attraction means causes the conveying belt to attract the sheet by an electrostatic attraction.
 5. A sheet reversing apparatus according to claim 4, wherein the attraction means includes control means which controls an attractive force according to a type of the sheet.
 6. A sheet reversing apparatus according to claim 1, further comprising auxiliary separation means which separates the sheet, attracted to the conveying belt, from the conveying belt at the conveying end position.
 7. A sheet reversing apparatus for reversing a sheet by a predetermined angle, comprising: a belt driving roller provided in a conveying start position of the sheet; a belt driving roller provided in a conveying end position of the sheet; an attraction belt supported in a state twisted by a predetermined angle between the belt driving rollers; and a charging control portion for controlling an AC power supply or DC power supply to supply the belt driving rollers with a current thereby providing the attraction belt with an electrostatic force.
 8. A sheet reversing apparatus according to claim 7, further comprising a backup roller at an approximate center of the belt driving rollers and inside the supported attraction belt, in order to prevent a mutual friction of the attraction belt.
 9. A sheet reversing apparatus according to claim 7, wherein the charge control portion controls the attractive force of the attraction belt according to a type of the sheet.
 10. A sheet reversing apparatus according to claim 7, further comprising a charge eliminating needle for reducing the attractive force of the attraction belt in the vicinity of the conveying end position of the sheet, whereby the sheet attracted to the attraction belt is separated by the charge eliminating needle.
 11. A sheet reversing apparatus according to claim 7, further comprising a separating claw rotatably provided in the vicinity of the conveying end position of the sheet and contacted with the attraction belt by a spring, whereby the sheet attracted to the attraction belt is separated by the separating claw.
 12. An image forming apparatus comprising: an image forming portion; and a two-side conveying path for conveying a sheet, which bears, on a first side thereof, an image formed by the image forming portion, to the image forming portion again; wherein the both-side conveying path includes a sheet reversing apparatus according to any one of claims 1 to
 11. 